Fossil pollen

Figure 11.8 Images of fossil pollen grains found in organic soil horizons from a range of plant species differentiated by shape and form using a high power microscope. Note an exotic spore is added to calculate the pollen concentration.

From the assemblages of fossil pollen, palynologists make inferences about the types of forests or other plant communities that may have occurred in the local environment. These interpretations must be made carefully, however, because species are not represented in the pollen record in ways that directly reflect their abundance as mature plants. For example, pollen of wind-pollinated species is relatively abundant in lake sediments, whereas species that are insect pollinated are not well... 

Palynology is the study of fossil pollen (and sometimes plant spores) extracted from lake sediment, peat bog, or other matrices. The most common goal of paly-nological research is to reconstruct the probable character of historical plant communities, inferred from the abundance of species in dated portions of the fossil pollen record. Pollen analysis is an extremely useful tool for understanding the character of ancient vegetation and its response to changes in environmental conditions, particularly in climate. Pollen analysis also has an economically important modem industrial use in the exploration for resources of fossil fnels. Palynology is also used to help reconstmct the probable habitats and foods of ancient humans and of wild animals. 

The pollen of many plants can be classified by genus, and sometimes by species, on the basis of such characteristics as size, shape, and surface texture, hi contrast, most spores can only be classified by higher taxonomic levels, such as family or order. Both pollen and spores are well preserved in lake sediment, peat bog, and many archaeological sites. Fossil pollen has even been identified from the bodies of extinct animals, such as mammoths discovered frozen in arctic permafrost (permanently frozen subsoil). 

A typical palynological study might involve the collection of one or more cores of sediment or peat from a site. The layers occurring at various depths would be dated, and samples of the pollen grains contained in the layers would be extracted, classified, and enumerated. From the dated fossil pollen of various species or genera, the palynologist would develop inferences about the nature of the forests and other plant communities that may have occurred in the local environment at the time. 

Wliile most researchers in the field agree on the relative importance of biospheric feedbacks operating at high northern latitudes, the discussion becomes more interesting and diverse as the subtropics are concerned. Climate reconstructions and data on fossil pollen compiled by Jolly et al. (1998), Hoelzmann et al. (1998), Pe-tit-Maire (1996), and Anhuf et al. (1999) indicate that North Africa was much greener in the mid-Holocene than today. The Saharan desert was, presumably to a large extent, covered by annual grasses and low shrubs. The Sahel reached at least as far north as 23 °N, more so in the western than in the eastern part. 

Additional information about seed dispersal is contained in the fossil record. Dispersal events can be inferred wherever there is evidence that populations were founded at large distances from the source population. It is, of course, technically difficult to detect a small population using fossil pollen or macrofossils, and even more difficult to demonstrate that the small population was isolated from the main population (Davis et al., 1991). Pollen studies in Sweden, however, record the establishment of individual colonies of beech (Fagus sylvatica) in the late Holocene (Bjorkman, 1996) and macrofossils demonstrate that populations of spruce (Picea abies) grew far in advance of the expanding species front for thousands of years (Kullman, 1996). East of James Bay, Canada, small colonies of larch (Larix laricina) have become established in patches during the past 1500 years as the population has expanded. Some of these colonies have fused into a continuous distribution. 

FIGURE 3 Maps showing distances between outlying colonies and the main population at the time the colonies were established, as indicated by the fossil pollen records of beech (a) and hemlock (b) (S. Webb, 1987 Woods and Davis, 1989 Davis et al., 1986 Calcote, 1986 Davis et al, unpublished data). 

Fossil pollen in a series of small forest hollows about 10 m in diameter provides a record of hemlock invasion of individual forest stands along a 10-km transect in northern Michigan. The distribution of species within the present-day forest, which has never been clearcut, is patchy—a mosaic of stands dominated by hemlock interspersed with mixed stands and large patches dominated by sugar maple (Acer saccharum). Pollen diagrams from four... 

Davis, M. B. and Botkin, D. B. 1985. Sensitivity of cool-temperate forests and their fossil pollen record to rapid temperature change. Q. Res. 23, 327-340. 

Webb, T III. 1974. A vegetational history from northern Wisconsin. Evidence from modern and fossil pollen. Am. Midland Nat 92,12-34. 

The science of reconstructing the past flora and past climate from pollen data obtained from lake and bog sediments. The fossil pollen record is a function of the regional flora and vegetation at a given time and location, particulate matter... 

Muller, J. (1981). Fossil pollen records of extant angiosperms. Botanical Review, 47,1-142. 

Fossil pollen was found worldwide, with the oldest occurrence being Calystegiapollis microechinatus from the early Eocene ( 55 my ago) of Africa (Martin 2001). The separation of Ipomoea nil (L.) Roth, imperial morning glory, and I. purpurea (L.) Roth, common morning glory, is assumed to be 8 my ago (late Miocene) (Durbin et al. 2001). 

Historical population migration at the rain forest-savanna boundary could also be addressed with molecular population genetics, although, in this case, studies should focus on rain forest species that are hypothesized to have recently expanded their distributions. If demographic expansion of rain forest species at the rain forest-savanna ecotone in areas such as Bolivia has been as rapid as the fossil pollen record implies (Mayle et al., 2000 Mayle, Chapter 17), then... 

The aim of this chapter, which expands upon Mayle (2004), is to review previously published palaeo-ecological data (predominantly fossil pollen records) from the South American tropics to determine how SDTF have responded to Late Quaternary enviromnental changes, and thereby better understand the development of the current disjunct pattern of dry forest distributions, and the floristic links between them. In particular, the validity of the dry forest refugia hypothesis, proposed by Prado and Gibbs (1993) and Pennington et al. (2000) will be examined. 

MacDonald, G.M. and L.C. Cwynar. 1985. A fossil pollen based reconstruction of the late Quaternary history of lodgepole pine Pinus contorta ssp. latifolia) in the western interior of Canada. Canadian Journal of Forest... 

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